Welcome itsmeek. Now that's a solid project to get your teeth into, but IMHO the GT4 is a fab funky beast and your efforts should give you lots of satisfaction in the end. 8)
I tend to agree with Ed, in that sitting exposed for 13 years may have had more impact on brake components than the more "sealed" environment within the hubs. It does seem extreme that ALL wheels are locked-up. Anyway, take the rims off and try to remove the brake calipers and pads. If the pistons have siezed this may be hard, as you won't be able to retract them by bleeding-off fluid. Once one pad is out you should be getting somewhere. Hopefully you can then spin the hub - if not, Pete is right and the brgs will need to be pulled.
Try to avoid stuffing the rotor or hub by getting too violent

as they will be expensive, while the brgs & brake bits should be more replaceable.
Re the hoot, I can only suggest closely examining the working catch. Has the RH cable (?) failed at the handle-end (& can you pull it with pliers)? Can you lever up the hood enough to see how the LH one works (and then work out how to tackle the RH one)?
My thoughts are that you will need lots of lateral thinking to get you to the end of the road you are travelling.
Anyway, GT4 (Peter) will have lots of suggestions soon ....
Good luck & keep us informed of the successes and problems.